Sidetracking of wells is a well-known procedure in which a new borehole is initiated at a small angle to an existing cased hole. The existing hole is closed by a packer below the intended sidetracking site, and a whipstock is secured on the packer to present an angled flat face towards the top of the well. A starting mill is then run into the well; the starting mill has a tapered nose to engage the angled face of the whipstock and side cutters which, when the string is rotated, cut an angled notch in the casing. The starting mill is then removed and replaced with a window mill. This is engaged in the notch and rotated to cut an elongate window through the casing and to form an initial length of the sidetrack bore.
The time taken to perform a sidetracking operation thus depends on the rate of penetration and reliability of the starting mill and the window mill. Mills used hitherto achieve only a modest performance. Starting mills require a high operating torque and tend to rotate unevenly or jam. Window mills have very poor penetration rate during part of the window-forming operation, to such an extent that they are frequently pulled because the operator suspects major damage to the tool but no such damage is found on recovery of the tool.